Internal welding head assembly adapted for welding at the interior of a structure, such as a pipe, tank, or the like, and adapted to be mounted upon a support which will move the welding head assembly along the weld track. The welding head assembly includes means for moving the welding tip with respect to the welding track so that the distance of the tip from the welding track may be adjusted, and includes welding tip oscillator means, welding current supply means, shielding gas supply means, operating gas supply means and means for guiding the welding tip along the weld track. A side plate mounts on the support for the welding head assembly. The welding head is pivotally connected to the support plate so that it may be moved with relation to the weld track. Means for oscillating the welding tip transversely of the weld track is provided in the assembly. The components of the welding head assembly are mounted compactly in a self contained package which may be readily transported, installed, removed, and replaced at the point of use. Since the assembly is small, it may be fitted within a small space, and plural welding head assemblies may be installed in the space previously occupied by a single welding head assembly. The apparatus provided according to this invention fulfills a need in the art, in providing such a welding head assembly which is compact, reliable, and simple in operation.